(a) Technical Field of Invention
This invention relates to thermal transfer printing and, in particular, to a thermal transfer printing receiver sheet for use with an associated donor sheet.
(b) Background of the Art
Currently available thermal transfer printing (TTP) techniques generally involve the generation of an image on a receiver sheet by thermal transfer of an imaging medium from an associated donor sheet. The donor sheet typically comprises a supporting substrate of paper, synthetic paper or a polymeric film material coated with a transfer layer comprising a sublimable dye incorporated in an ink medium usually comprising a wax and/or a polymeric resin binder. The associated receiver sheet usually comprises a supporting substrate, of a similar material, having on a surface thereof a dye-receptive, polymeric receiving layer. When an assembly, comprising a donor and a receiver sheet positioned with the respective transfer and receiving layers in contact, is selectively heated in a patterned area derived, for example--from an information signal, such as a television signal, dye is transferred from the donor sheet to the dye-receptive layer of the receiver sheet to form therein a monochrome image of the specified pattern. By repeating the process with different monochrome dyes, usually cyan, magenta and yellow, a full coloured image is produced on the receiver sheet. Image production, therefore depends on dye diffusion by thermal transfer.
To facilitate separation of the imaged sheet from the heated assembly, at least one of the transfer layer and receiving layer may be associated with a release medium, such as a silicone oil.
Although the intense, localised heating required to effect development of a sharp image may be applied by various techniques, including laser beam imaging, a convenient and widely employed technique of thermal printing involves a thermal print-head, for example, of the dot matrix variety in which each dot is represented by an independent heating element (electronically controlled, if desired).
Available TTP print equipment has been observed to yield defective imaged receiver sheets comprising inadequately printed spots of relatively low optical density which detract from the appearance and acceptability of the resultant print. These small defective areas, conveniently referred to as micro-dots, are believed to result from poor conformation of the donor sheet to the print-head at the time of printing.
(c) The Prior Art
Various receiver sheets have been proposed for use in TTP processes. For example, EP-A-0194106 discloses a heat transferable sheet having a substrate and an image-receiving layer thereon, with an intermediate layer between the substrate and receiving layer.
The intermediate layer serves as a cushion between the substrate and receiving layer and consists mainly of a resin, such as a polyurethane, polyacrylate or polyester, having a 100% modulus of 100 kg/cm.sup.2 or lower, as defined by JIS-K-6301. Inadequate adhesion between the donor and receiver sheets is observed if the intermediate layer is formed from a resin of higher modulus.
U.S. Pat. No. 4734397 seeks to avoid the production of irregular images resulting from entrapment of dust and non-uniformity of the dye-receptive layer by providing a receiver sheet comprising a compression layer between a substrate and a dye-receptive layer. The compression layer, which preferably comprises a resin, such as polymethylmethacrylate, an acrylonitrile-styrene copolymer, a modified polybutylene-terephthalate or a polyurethane, is applied to the substrate as a coating, for example--as a solution in a mixed solvent comprising dichloromethane and trichloroethylene, at a coverage of at least 2.0 g/m.sup.2 and has an elasticity of less than 500% elongation at break. Preferably, the compression layer exhibits a compression modulus of less than 350 megaPascals.
Additional processing and drying procedures are involved in the provision of a compression coating layer. In addition, the presence of such a layer is liable to interfere with dyes transferred into the adjacent receiving layer, thereby inducing undesirable variations in the shade pattern of the resultant image.
We have now devised a simplified receiver sheet for use in a TTP process which overcomes or substantially eliminates the aforementioned micro-dot problem, without the need for an additional compression layer.